This invention relates generally to axle driving apparatus and, more particularly, to a transaxle and hydrostatic transmission assembly having an improved hydraulic fluid bypass mechanism.
Transaxle and hydrostatic transmission assemblies (xe2x80x9cHSTsxe2x80x9d) are known in the art. Generally, an HST includes a center section on which is mounted a hydraulic pump and a hydraulic motor. The hydraulic pump and the hydraulic motor each carry a plurality of reciprocating pistons which are in fluid communication through hydraulic porting formed in the center section. Rotation of the hydraulic pump by an engine creates an axial motion of the pump pistons which forces an operating oil through the hydraulic porting to the hydraulic motor to move the motor pistons. The axial motion of the motor pistons causes the hydraulic motor to rotate as the motor pistons bear against a thrust bearing. In this manner, the rotation of the hydraulic motor may be used to drive the vehicle axles of a riding lawn mower, small tractor and the like.
For allowing the vehicle to roll or freewheel without resistance from the hydraulic fluid, it is also known in the art to include a hydraulic fluid bypass mechanism in an HST. By way of example, U.S. Pat. Nos. 5,201,692 and 5,819,537, which are incorporated herein by reference in their entirety, disclose a hydraulic fluid bypass mechanism that functions by lifting the hydraulic motor off the motor running surface of the center section to break the hydraulic fluid circuit that circulates operating oil between the hydraulic pump and the hydraulic motor. To activate this feature, a bypass arm is manipulated to rotate a bypass actuator. The bypass actuator includes a rod which is shaped with a flat surface formed along a part of the outer periphery of its lower end. Rotation of the rod forces the corner of the flat surface to press a plate or pin against the base of the hydraulic motor to thereby break the hydraulic seal between the hydraulic motor and the center section.
To prevent damage to the components that comprise the hydraulic motor, in particular, to the piston springs, the hydraulic fluid bypass mechanisms of the types disclosed in U.S. Pat. Nos. 5,201,692 and 5,819,537 require the rotational movement of the actuator rod to be limited. Limiting the rotational movement of the actuator rod is required as it limits the distance into the space of the plate or pin the comer of the flat surface of the actuator rod travels which, in turn, limits the distance into the space of the hydraulic motor that the plate or pin travels. For this purpose, the hydraulic fluid bypass mechanism disclosed in U.S. Pat. No. 5,201,692 uses a stop formed on the actuator rod to place a limit on the rotational movement of the actuator rod. Similarly, although not illustrated, the hydraulic fluid bypass mechanism disclosed in U.S. Pat. No. 5,819,537 uses externally mounted pins that cooperate with an actuator handle to stop the movement of the actuator rod. In accordance with either approach, the movement of the actuator rod is limited such that the flat surface of the actuator rod engages the plate or pin at an angle generally less than 90 degrees at the point at which the corner achieves the maximum allowable displacement of the plate or pin.
While these known hydraulic fluid bypass mechanisms do work for their intended purpose, they suffer the disadvantage of requiring a manual or mechanical means by which the actuator rod may be maintained in a position that results in the breaking of the hydraulic circuit. Since the rotational movement of the rod is limited and the flat surface of the rod meets the plate or pin at an angle, the actuator rod is subjected to a restoring torque as the hydraulic motor assembly, which is biased by its piston springs, seeks to return to the position where it sits flush upon the motor running surface of the center section. Thus, the manual or mechanical means is required to maintain the position of the actuator rod against the restoring torque that would otherwise force the actuator rod to a position that does not create a break in the hydraulic circuit.
To eliminate the effects of restoring torque on a mechanical bypass mechanism, an improved mechanism for bypassing the flow of hydraulic fluid in a hydrostatic transmission is provided. The mechanism includes a bypass actuator rod that is moved from a first position to a second position to thereby move a plate against the hydraulic motor to lift the hydraulic motor off of the center section. The bypass actuator rod has a generally flat surface and an opposite arcuate surface wherein the generally flat surface of the bypass actuator is positioned generally adjacent and parallel to the plate in the first position and the arcuate surface of the bypass actuator engages the plate in the second position. When the bypass actuator rod is in the first position the hydraulic motor is in substantial sealed engagement with the center section. When the bypass actuator rod is in the second position the hydraulic motor is lifted off of the center section and any forces applied to the actuator rod by the lifted hydraulic motor are directed substantially towards the axis of the bypass actuator rod.
A better understanding of the objects, advantages, features, properties and relationships of the invention will be obtained from the following detailed description and accompanying drawings which set forth an illustrative embodiment and which are indicative of the various ways in which the principles of the invention may be employed.